Accurate collecting of point-blank passing time of an airplane passing through the overhead sky at one spot on the ground is an essential basic requirement for increasing measurement accuracy when various measurements are performed on the airplane.
In conventional measurement of point-blank passing time, point-blank passing time is usually estimated by removing noise components from data on changes in sound noise level, obtained by measuring airplane-produced sound noise, with reference to the time when the peak value of the change of sound noise level is collected, and further by taking into account the weather and meteorological conditions and the like.
Further, in measuring the above described airplane sound noise, by receiving a transponder response signal radio wave of 1090 MHz which is emitted from the airplane and non-directional in horizontal plane, and by examining the correlations between the changes in the electric field intensity level of the radio wave and the airplane model identification data and flight height data obtained by decoding the transponder response signal, it has been possible to accurately measure the point-blank passing time based on the time when the peak value of electric field intensity is collected (Japanese Patent Laid-Open No. 4-40646).
In a conventional method for measuring point-blank passing time according to the above described airplane-produced sound noise measurement, the method requires taking into account complicated variable factors to perform a cumbersome task of data analyses, and further the measurement accuracy is insufficient and thus not completely satisfactory.
Also, in a method for measuring the point-blank passing time according to both the sound noise measurement and the peak value of the electric field intensity level of the transponder response signal radio wave, the measurement can be performed with considerable accuracy. However, the peak value of the transponder response signal radio wave itself is flat in time distribution, thus causing a problem that sufficient accuracy can not be achieved.
Recent airplane flights are significantly increased in number, and particularly the number of airplanes taking off from and landing to an airport can be more than 500 per a day. In the surroundings of the airport, flight density can be so high that the flight interval may be less than 90 seconds, resulting in serious pollution of airplane-produced sound noise.
In the case of such a high frequency of flight, the measuring of point-blank passing time according to the above described sound noise-measuring means is difficult to identify individual airplanes and therefore virtually produce no effect. Also, the means for receiving the transponder response signal radio wave has a problem that it is almost impossible to collect the peak value of electric filed intensity of the radio wave.